Hi all - new here, lurked for awhile, now asking questions :-)  I just posted in the SIPs area regarding some questions I have re: SIPs roof treatments.  I also have a question regarding wall construction:

I'm going to owner/builder my house - I'm a civil engineer, so I feel comfortable doing this.  Anyways, I spoke with a local energy star builder who likes 2x8" 24 OC walls with wet spray cellulose.  Has anyone here used this assembly?  I have concerns over the ability of 7 1/2 of wet spray cellulose to dry in any reasonable amount of time before vapor retarder/sheetrock is placed over it.  I also have concerns over the eventual settling of the cellulose in the wall.  I realize the spray variety sets up a bit to be more rigid, but I dunno, it still bothers me.  Also, thermal transfer through the studs is a concern to me. 

I guess my question is, anyone here have any guidelines/advice/insight into wet spray cellulose walls?  I wonder if I do this wall and then place foamboard on the outside for a thermal break, I might end up the same price as a SIPs wall - any guesses as to that? 

Thanks!!

Rich

Rich,
Check out the Oak Ridge National Laboratories site for the testing of 'whole wall R values'.

Thanks Wes - I did check ORNL, they only have a 2x6 wall, it shows a whole wall R of 14.2 or so w/2x6 24" OC with cellulose and foam sheathing. They have VERY limited SIPs data that indicates a whole wall R of 16 or so with a 6.5" EPS SIP - I'm not sure I put alot of stock in that calculator.

We used that product on my last house. Yes, even in the dry mountains, it does take a while to dry. It was supposed to seal the walls. But, there was air leaking through at many places in the exterior walls.

If I were to do it again, I would use SIPS. Spray foam would work, but SIPS are actually less expensive when you consider the framing costs(labor & materials) and add in the spray foam costs.

jc,
would I be correct to think that the cellulose shrinks away from the framing slightly as it dries, which allows for the possiible air leakage?

Rich,
The numbers I have from ORNL indicate a 2x6 wall R value of 13.69, and 3.5" core SIPS at 13.86.

Wes,
I suppose that's possible, but I didn't have a chance to see it. After a few days, the insulators came in and added fiberglass to the cellulose, and sealed it with poly(plastic).

I think that the problem is that is was only about 2" thick(in a 2x6 wall), and it didn't cover the holes made by the electricians and plumbers. And, they covered the cellulose with fiberglass, and that doesn't stop the air from flowing thru those holes.

just heard about Sips and have been doing research on it but I can't find a general rule of thumb that installers charge per square foot, particularly just for exterior walls. Does anyone have any info. or advice of where to look. I can't find any installers listed in my area.

I got one estimate of about 12$ per square ft of wall space, someone please correct me if I am wrong

See if anyone in your area does dense pack cellulose - its blown in at density at which it won't settle & you don't have to wait for it to dry. That plus 2" of extruded polystyrene will give you an R-36. By the way, don't use the poly.

if you have to go 2/8 the extra cost of custom door and window jambs pluss other additional hassles would seem to me to make sips
a no brainer. remember its not how thick your walls are thats important it how tight they are.

Bob - why exactly would you recommend to not use the poly?

I'm specifically repeating the advice of the Building Science Corp - building scientists and forensic architects who have been studying the whole range of construction and insulation issues that have arisen in the last 30-40 years. Just took a one day seminar they put on specifically discussing insulation types, problems, what works, what works better etc. They've learned from experience in building forensics that walls have be built to mimize air infiltration and be able to dry to one side, the other side or both sides. Obviously this is only one in thousands of important details. Visit the web site - buildingsciencecorporation.com. Go to Seminars then Whats New for presentations from recent seminars. We're all trying to figure this stuff out & these are the experts.